Synthesis and Effect of Encapsulating Rejuvenator Fiber on the Performance of Asphalt Mixture
2. Materials and Test Methods
2.1. Gradation Design of Asphalt Mixture
AC-13 asphalt mixture was designed in this work. Aggregate gradation is shown in Table1.
70# bitumen was used in this work, and its penetration (20◦C, 0.1 mm) and softening point were 68.7 and 48.5◦C respectively. Its ductility (15◦C) was larger than 100 cm. Basalt was used as aggregate.
The optimum bitumen content and the content of the fiber were 4.7% and 0.235%, respectively, based
on Marshall design method. air void (VV), voids in mineral aggregate (VMA) and voids filled with asphalt (VFA) were 3.9%, 14.5% and 73.1%, respectively.
Table 1.Aggregate gradation of AC-13 asphalt mixture.
Sieve Size/mm Designed Gradation/%
16 100
13.2 96.2
9.5 75.2
4.75 47.4
2.36 30.8
1.18 23.9
0.6 16.6
0.3 12.3
0.15 9.1
0.075 6.9
2.2. Synthesis of the Three Kinds of Fibers
Alginate, dehydrated calcium chloride, silica (SiO2) nanoparticles, graphene oxide (GO) and asphalt rejuvenator were purchased from SINOPHARM GROUP CO. LTD. Asphalt rejuvenator was mainly composed of 67.4% aromatics and 21.07% saturates.
Alginate solution (2 weight% alginate) was prepared to synthesize calcium alginate fiber encapsulating asphalt rejuvenator. Alginate/SiO2 (alginate: SiO2= 1:1) solution was prepared to synthesize calcium alginate/SiO2composite fiber encapsulating rejuvenator. Alginate/GO (alginate:
GO=1:10) solution was prepared to synthesize calcium alginate/GO composite fiber encapsulating rejuvenator. The purpose of synthesizing the three kinds of fibers is that: the synthesis of calcium alginate/SiO2composite fiber is going to improve thermal and mechanical properties, and decrease the leakage of calcium alginate fiber; the synthesis of calcium alginate/GO composite fiber is going to combine the advantages of the induction heating method and the capsule method. The fiber can be heated by microwave to increase the temperature of asphalt, so as to achieve the purpose of quickly healing cracks inside asphalt. Further, the asphalt can be rejuvenated by the encapsulated rejuvenator.
Therefore, calcium alginate/GO composite fiber can make the cracks inside the asphalt double healed.
The three kinds of fibers were synthesized by using a self-assembled microfluidic device.
The synthetic process was shown in a previous study [23].
2.3. Characterization of the Three Kinds of Fibers
Morphology of the three kinds of fibers was characterized by scanning electron microscope (SEM) test. SEM test was conducted on an S4800 machine. Before SEM test, the specimens were sprayed with gold for 50 s.
Chemical structure of the three kinds of fibers was evaluated by Raman spectra test. An InVia instrument with a wavelength ranging from 200 to 2000 cm−1was used to complete Raman spectra test.
Thermal properties of the three kinds of fibers were tested by thermogravimetric analyzer (TGA) experiment. The testing temperature raised from 30 to 600◦C with a heating rate of 10◦C/min.
2.4. Road Performances of Asphalt Mixture with the Fibers
Rutting test, water stability test, freeze-thaw splitting test and low temperature bending test of asphalt mixture were carried out according to the specification “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering JTG E20-2011”.
Rutting test was conducted on a YLDCZ-6S machine with the testing temperature of 60◦C.
The wheel speed was 21 times/min, and loading was 0.7 MPa. The length, width and height of
specimen were 300 mm, 300 mm and 50 mm, respectively. Before the testing, the sample was kept 5 h at 60◦C. The dynamic stability (DS) was defined as follows:
DS=15×N/(d60−d45) (1)
where N=42 cycles/min, which means the wheel tracking speed;d60andd45are the rutting depth at the wheel tracking time of 45 min and 60 min, respectively.
The low temperature crack resistance of asphalt mixture was evaluated by flexural strength, flexural strain and flexural modulus. The length, width and height of specimen were 250 mm, 30 mm and 35 mm, respectively. The test temperature was−10◦C and the loading rate was 50 mm/min.
The water stability of asphalt mixture with the fibers was evaluated by freeze-thaw split strength ratio (TSR). For that, the specimen was firstly freezing at−18◦C for 16h, and then suffered a water bath for 24 h at 60◦C.
TSR=TS2
TS1×100% (2)
whereTS2was splitting tensile strength of the specimen after freeze-thaw experiment;TS1was splitting tensile strength of the specimen before freeze-thaw experiment.
Four-point bending fatigue test was carried out to study the fatigue performance of asphalt mixture containing the fiber. Beams with length, width and height of 400 mm, 50 mm and 50 mm respectively were used. The test temperature and working frequency were 20◦C and 30 Hz respectively.
2.5. Self-Healing Capability of Asphalt Mixture with the Fibers
Three-point bending test on the asphalt mixture beams with length, width and height of 95 mm, 40 mm and 50 mm respectively, was performed by using a universal testing machine (UTM-25). Before the test, the specimens were kept at−10◦C for 4 hours. Then the specimens were tested at−10◦C with a loading rate of 0.5 mm/min. After that, the two parts of a tested specimen were rejoined by three rubber bands. Healing temperature and healing time were 30◦C and 3 days respectively. A microwave oven with the power of 800 W was used to complete microwave heating test. The self-healing ability of each asphalt mixture beam was quantified by the Healing Index (HI) according to Equation (3).
HI= Fr
Fi
×100 (3)
where,Fris recovery of peak strength of an asphalt beam after healing rest, kN;Fiis the initial peak strength of the same asphalt beam, kN.
Types symbology of fibers and asphalt mixtures is shown in Table2. A1 was the asphalt mixture without fiber. Three sets of replicate samples were tested in each experiment.
Table 2.Types symbology of fibers and asphalt mixtures.
Types of Fiber Symbology Types of Asphalt Mixture Symbology
Ca–alginate fiber AF Asphalt mixture containing
Ca–alginate fiber A2
Ca–alginate/SiO2fiber ASF Asphalt mixture containing
Ca–alginate/SiO2fiber A3
Ca–alginate/GO fiber MRF Asphalt mixture containing
Ca–alginate/GO fiber A4
3. Results and Discussion
3.1. Characterization of Three Kinds of Self-Healing Fibers 3.1.1. Morphology
Figure1shows the synthesis process and synthesis mechanism of the three kinds of self-healing fibers. Alginate is composed of a series of G units and M units. Sodium ions on G units can be rapidly replaced by calcium ions to form calcium alginate gels. The whole reaction process is completed in an instant, so, nano SiO2particles and graphene oxide particles can be completely incorporated into calcium alginate gel. The three kind of self-healing fibers, which include Ca–alginate fiber, Ca–alginate/SiO2composite fiber and Ca–alginate/GO composite fiber were thus fabricated.
Figure 1.Synthesis process (a) and synthesis mechanism (b) of the three kinds of fibers.
The morphology of three kinds of self-healing fibers is shown in Figure2. It can be seen from Figure2a–c that AF and ASF had a light-yellow optical appearance, while MRF had a black appearance.
From Figure2(a1–c1), it can be observed that the wall of three kinds of fibers was intact and there were no visible holes and cracks, which indicated that rejuvenator could be completely encapsulated inside the fibers, and without leakage. The diameter of the fibers was about 500μm. Figure2(a2–c2) reveal that the surface of AF was the smoothest. ASF fibers had an uneven surface with many small bulges, which was attributed to the incorporation of partially agglomerated SiO2particles. A large number of regular folds appeared on the surface of MRF, which were peculiar to the specific structure of graphene.
Over all, the three kinds of self-healing fibers encapsulating rejuvenator were successfully synthesized from the analysis of morphology.
Figure 2.Morphology of the three kinds of self-healing fiber: Optical morphologyof Ca-alginate fiber (a), Ca-alginate/SiO2fiber (b) and Ca-alginate/GO fiber (c); Micromorphology of Ca-alginate fiber (a1,a2), Ca-alginate/SiO2fiber (b1,b2) and Ca-alginate/GO fiber (c1,c2).
3.1.2. Chemical Structure
Raman test was conducted to confirm the successful synthesis of the three kinds of self-healing fibers. Figure3a revealed that GO had two characteristic peaks, namely peak D and peak G.
The two peaks didn’t appear in the Raman spectra of calcium alginate fiber, while appeared in Ca–alginate/GO fiber Raman spectra. A characteristic peak at 1070 cm−1appeared in both Ca–alginate fiber and Ca–alginate/GO fiber. The three kinds of characteristic peaks appeared in the Raman spectra of Ca–alginate/GO fiber, which indicated that Ca–alginate/GO fiber was successfully synthesized.
Similarly, SiO2had a series of characteristic peaks in the Raman shift from 200 cm−1to 800 cm−1in Figure3b. These peaks also appeared at the same wavelengths in Raman spectrum of Ca–alginate/SiO2
fiber, which revealed that SiO2was successfully incorporated inside the Ca–alginate structure, and Ca–alginate/SiO2composite fiber was successfully synthesized.
Figure 3. Raman test of the three kinds of fibers: (a) Ca–alginate fiber and Ca–alginate/GO fiber;
(b) Ca–alginate/SiO2fiber.
3.1.3. Thermal Stability
The thermal stability of three kinds of self-healing fibers was studied by TGA test and the results is shown in Figure4. It can be seen that the initial decomposition temperature of rejuvenator was about 250◦C, which means that the rejuvenator meet the temperature requirement in the mixing and compaction process of asphalt mixture. Between 100◦C and 180◦C, the loss mass of Ca–alginate self-healing fiber was attributed to degradation of crystalline water and other components of calcium alginate wall. The decomposition mass and decomposition rate of ASF and MRF were both significantly lower than those of AF, which indicated that the addition of nano-SiO2and GO particles could improve the thermal stability of Ca–alginate fiber. The loss mass of fiber between 200 and 400◦C was mainly attributed to the decomposition of rejuvenator and calcium alginate materials. Based on the residual mass at 600◦C, the relative mass of encapsulated rejuvenator could be calculated out. The relative mass of rejuvenator that was encapsulated in AF, ASF and MRF were 56.65%, 47.25% and 32.94%
respectively. Based on the above analysis, the three kinds of self-healing fiber meet the temperature requirement in the mixing and compaction process of asphalt mixture.
Figure 4.Thermal stability of the three kinds of fiber.
3.2. Road Performance of the Asphalt Mixture with Fibers 3.2.1. High-Temperature Performance
The high temperature performance of asphalt mixture containing the different kinds of fiber was investigated by wheel tracking test and the result is shown in Figure5. Dynamic stability (DS) and rutting depth (RD) were tested to evaluate high temperature anti-rutting ability of asphalt mixture. The larger DS and smaller rutting depth, the better anti-rutting ability asphalt mixture has. It can be seen that the addition of fibers led to an improved DS and decreased rutting depth. For example, the DS of A2, A3 and A4 increased from 2450 times/mm to 3014 times/mm, 3223 times/mm and 3127 times/mm respectively. The rutting depth of A2, A3 and A4 decreased from 2.415 mm to 1.831 mm, 1.672 mm and 1.698 mm respectively. The improved anti-rutting ability may be attributed to that the entanglement of fibers in asphalt mixture reduced the content of free asphalt.
Figure 5.DS and rutting depth of asphalt mixture with the fiber: DS (a) and RD (b) of asphalt mixture.
3.2.2. Low Temperature Performance
Low temperature bending test was conducted, and flexural strength, flexural strain and flexural modulus were obtained to evaluate low temperature anti-crack ability of asphalt mixture containing the fiber. Figure6a revealed that the addition of the fiber increased flexural strength of asphalt mixture.
The flexural strength of A2, A3 and A4 increased from 9.28 MPa to 10.23 MPa, 10.68 MPa and 10.01 MPa, respectively. It can be seen from Figure6b that the flexural strain of asphalt mixture containing the fiber was slightly smaller than that of asphalt mixture without fibers. Nevertheless, the flexural strains of all asphalt mixtures containing the fiber were larger than 2800με, which indicates that the flexural strain of asphalt mixture containing the fiber meets the requirements. In addition, the addition of the fiber could improve the flexural modulus of asphalt mixture. The flexural modulus of A2, A3 and A4 increased from 2968 MPa to 3325 MPa, 3518 MPa and 3447MPa, respectively.
Figure 6.Flexural strength (a), Flexural strain and flexural modulus (b) of asphalt mixture containing the fiber.
3.2.3. Water Sensitivity
Freeze-thaw splitting test was performed to evaluate the water sensitivity of asphalt mixture containing the fibers, and the result is shown in Figure7. It can be observed that the TSR of asphalt mixture slightly decreased when the fiber was added. In details, the TSR of A2, A3 and A4 decreased from 89.7% to 82.5%, 84.3% and 83.6%, respectively. The slightly decreased TSR may be attributed to the relatively strong hydrophilicity of the fiber. Nevertheless, the TSR of asphalt mixture with the fiber was still larger than 75%. Thus, moisture stability of asphalt mixture with the fiber met the requirement (according to the specification “Technical Specification for Construction of Highway Asphalt Pavements JTG F40-2004”).
Figure 7.TSR of asphalt mixture with the fiber.
3.2.4. Fatigue Performance
Four-point bending fatigue test was conducted to evaluate fatigue performance of asphalt mixture with the fiber. It can be seen from Figure8that there was no obvious difference in initial stiffness for asphalt mixture with, and without the fiber, which meant that the fibers could remain intact in asphalt mixture after the mixing and compaction process. The stiffness of asphalt mixture with the fiber was smaller than that of asphalt mixture without the fiber from 10,000 numbers of load cycles to 60,000 numbers of load cycles, which indicated that the fiber was ruptured, rejuvenator that was encapsulated in the fiber flowered out, and then rejuvenated and softened bitumen. Fatigue failure of asphalt mixture without the fiber appeared when the number of load cycles was larger than 60,000.
While due to the regeneration of rejuvenator, asphalt mixture with the fiber could continue to withstand fatigue loading. Thus the addition of the fibers could prolong the fatigue life of asphalt mixture.
Figure 8.Fatigue performance of asphalt mixture with the fibers.
3.3. Self-Healing Capacity of the Asphalt Mixture with Fibers
The self-healing capacity of asphalt mixture with the fiber was tested by 3PB experiment and the result is shown in Figure9. As shown in Figure9, the HI of all fiber containing asphalt mixtures increased to varying degrees. Under the rejuvenation effect of rejuvenator, the HI of A2, A3 and A4 increased from 50.7% to 68.8%, 65.3% and 61.4%, respectively. Under the synergistic action of microwave heating and rejuvenator, the HI of A2, A3 and A4 increased from 53.2% to 74.5%, 72.1% and 89.2%, respectively. The result revealed that the addition of three kinds of fiber could improve the
self-healing ability of asphalt mixture under the action of rejuvenator that was encapsulated in fiber.
Furthermore, because of the incorporation of microwave absorbent (GO) into self-healing fiber, under the synergistic action of microwave heating and rejuvenator, the self-healing ability of asphalt mixture containing MRF could be significantly improved. GO is an excellent microwave heating material that generates a large amount of heat under the action of microwave heating. As the temperature of the asphalt increases rapidly, the fluidity of the asphalt is greatly improved. Under the action of the rejuvenator and temperature, the crack inside the asphalt mixture will be quickly filled by the flowing asphalt, so the self-healing properties of the asphalt mixture containing Ca–alginate/GO fiber are significantly improved after microwave heating (as shown in Figure9A4).
Figure 9.Self-healing capacity of asphalt mixture containing the fiber.
4. Conclusions
In this study, three kinds of self-healing fiber were synthesized. A series of characterizations were performed to study the morphology, chemical structure and thermal stability of the fibers. Then, road performance and self-healing capacity of asphalt mixture containing the fiber were investigated. Based on the tests and results, the follow conclusions can be drawn:
(a) Ca–alginate fiber, Ca–alginate/SiO2composite fiber and Ca–alginate/GO composite fiber were successfully synthesized by microfluidic device. Nano SiO2and GO particles were incorporated into the fiber wall. Rejuvenator was encapsulated inside those fibers in the form of droplets.
The three kinds of fibers had excellent thermal stability, which meet the temperature requirement in the mixing and compaction process of asphalt mixture.
(b) The addition of the three kinds of fibers improved high temperature anti-rutting ability of asphalt mixture. In addition, those fibers increased flexural strength and flexural modulus, while slightly decreased flexural strain of asphalt mixture. Moisture stability of asphalt mixture containing the fiber had a slight decrease. In addition, the fibers could prolong fatigue life of asphalt mixture under the action of encapsulated rejuvenator. In short, the road performances of asphalt mixture containing the fiber meet the requirements.
(c) The self-healing ability of asphalt mixture with fiber was better than that of asphalt mixture without the fiber. It was worth noting that the synergistic action of microwave heating and rejuvenator could further significantly improve the self-healing ability of asphalt mixture.
This study provides the possibility of industrial production of self-healing asphalt mixtures.
Different types of self-healing fibers can be fabricated in large scale by this method. Personally, I believe
that the future trend is a combination of capsule method and induction method. Combining the advantages of capsules method and induction heating method to achieve a substantial improvement in the self-healing properties of asphalt concrete. As in this work, the composite Ca–alginate/GO fibers with crack response behavior and microwave heating response behavior combined the advantages of the capsules method and induction heating method. The fiber not only can rejuvenate the asphalt, but also can rapidly increase the temperature of the asphalt by microwave heating, which quickly increased the fluidity of asphalt, and thus the self-healing properties of the asphalt mixture can be quickly improved under the synergistic effect of the rejuvenator and the temperature.
Highlights
• Three kinds of fibers encapsulating asphalt rejuvenator were synthesized;
• Road performance of asphalt mixture containing the fiber met the requirements;
• Three kinds of fibers could improve self-healing ability of asphalt mixture.
Author Contributions: Conceptualization, B.S. and Q.L.; Data curation, B.S. and S.B.; Formal analysis, B.S.;
Investigation, B.S.; Methodology, B.S., C.L., L.D. and Q.W.; Writing–original draft preparation, B.S.; Writing–review and editing, Q.L., X.Y. and J. N.; Project administration, S.W.; Supervision, S.W. and Q.L.
Funding:This research was funded by National Key R&D Program of China (No. 2018YFB1600200). This work presented in this paper was supported by National Natural Science Foundation of China (71961137010). The authors acknowledge the financial support by the Natural Science Foundation of China (No. 51778515) and National Key R&D Program of China (No. 2017YFE0111600). Finally, the sixth author thanks the financial support by the National Science Foundation of China (No. 5170081212).
Conflicts of Interest:There are no conflicts of interest regarding the publication of this paper.
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